1. Field of the Invention
The present invention relates to an acoustic apparatus and, more particularly, to a compact acoustic apparatus which has clear sound localization and a novel outer appearance.
2. Description of the Prior Art
FIGS. 5A and 5B are respectively a elevation view and a sectional view showing an arrangement of the phase-inversion (bass-reflex) speaker system as one conventional acoustic apparatus. In the speaker system shown in FIGS. 5A and 5B, a hole is formed in the front surface of a cabinet 1, a vibrator (speaker unit) 4 consisting of a diaphragm 2 and a dynamic converter (speaker) 3 is mounted in the hole, and an open port 5 (so-called a bass-reflex port or resonance port) is formed therebelow. The open port 5 and the cavity of the cabinet 1 form a Helmholtz resonator. The open port 5 is formed into a tubular shape by a paper, plastic or wood material. Note that the open port 5 may be arranged on the rear surface of the cabinet 1, as shown in FIG. 6.
In the bass-reflex speaker system according to the conventional basic setting, a resonance frequency (Helmholtz resonance frequency f.sub.OP defined by an air spring of the cabinet 1 and an air mass in a sound path 6 of the open port 5 is set to be lower than a lowest resonance frequency f.sub.O of the vibrator 4 when the vibrator is assembled in the bass-reflex cabinet 1. At a frequency higher than the resonance frequency f.sub.OP defined by the air spring and the air mass, the phase of sound pressure from the rear surface of the diaphragm 2 is inverted at the open port 5. Consequently, in front of the cabinet 1, a sound directly radiated from the front surface of the diaphragm 2 is in phase with a sound from the open port 5 by resonance, and the sound pressure is increased. As a result, according to an optimally designed bass-reflex speaker system, the frequency characteristics of the output sound pressure can be expanded to the resonance frequency f.sub.O of the vibrator 4 or less. As indicated by an alternate long and two short dashed curve in FIG. 7, a uniform reproduction range can be widened as compared to an infinite plane baffle or closed baffle.
However, in the conventional bass-reflex speaker system, since independent spaces for respectively arranging the vibrator and the open port are set, a space factor is poor, and the speaker system cannot be easily reduced in size. In addition, a directly radiated sound from the vibrator and a resonantly radiated sound from the open port tend to serve as two sound sources, resulting in unclear sound localization.
An interval between the diaphragm and the open port is relatively small. For example, if this interval is smaller than a value three times an effective radius of the diaphragm of the vibrator, in-phase sounds radiated from the vibrator and the open port cancel each other, and hence, frequency characteristics, in particular, bass-sound characteristics are adversely influenced.